This invention relates to a method of reading a pattern such as a bar cord pattern which is useful in postal service, in distribution business or in management of confidential documents or articles, and also relates to an optical signal reader for reading signal from such a pattern.
In recent years, a system for automatically inputting information by making use of bar code has been studied in various fields of industry. For example, there has been an attempt to print bar code corresponding to zip code and address on the surface of mails such as postal cards so as to allow to sort mails automatically by reading the bar code on the mails, thereby enhancing the efficiency of postal service. There has been also proposed a system in hospital wherein bar code corresponding to medical chart is printed on the surface of a consultation ticket for the patient so that as soon as the bar code on the consultation ticket is read by bar code reader, the medical chart can be automatically transferred from file stacks to a doctor. Furthermore, the application of bar code is also studied for an automatic retrieval system of official documents in government office, for a management system for the entry and delivery of clothes in dry-cleaning business, for an automatic retrieval system of training record from a training card in driver's school, or for a storage system of various goods.
In the application of bar code for these systems, various problems are raised if the bar code is printed with the conventional black ink. For example, since characters indicating address are already written on the surface of mail, if the bar code array newly printed on the surface of mail is overlapped with these characters, the signals to be detected from the bar code may be accompanied with noise, thus making it difficult to read information correctly from the signals. This problem may become more serious when the length of the bar code array becomes longer for recording much information. Moreover, the black bar code printed on the mail not only spoils the appearance of the mail but may make the characters already written thereon unreadable.
Even if a color ink containing orange, blue or brown dye is employed in place of black ink for printing bar code, the same problem as in the case of bar code made of a black ink would be raised if the colored bar code is overlapped with the characters already written on the mail. Further, there are also problems that some of dyes are harmful to human body and that many dyes are easily subjected to photo-degradation.
With a view to solve these problems, novel material for forming bar code has been studied. For example, the employment of a fluorescent material which is capable of absorbing ultraviolet light and emitting fluorescence in visible region is studied to form transparent bar code. In this case, the region including bar code is irradiated with ultraviolet light as excitation light to cause fluorescence in visible region to emit, and the resultant fluorescence is detected as signals. Since the bar code in this case is transparent in the ordinary conditions, the readability of the characters already written on the mail would not be hindered even if the bar code is overlapped with the characters. However, there is another problem that since white paper used for postal cards, for instance, generally contains fluorescent whitening agent, noise of considerable magnitude as compared with the signal from the bar code is detected from the paper, whereby deteriorating the S/N ratio.